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A347885 Odd numbers k such that sigma(k^2) has an odd number of prime factors when counted with multiplicity. 4
3, 5, 17, 21, 27, 33, 35, 37, 39, 41, 45, 49, 55, 57, 59, 61, 65, 69, 71, 75, 87, 89, 93, 95, 101, 107, 109, 115, 119, 125, 129, 131, 137, 139, 141, 145, 149, 151, 153, 155, 159, 167, 169, 173, 181, 187, 189, 193, 201, 215, 219, 221, 229, 231, 235, 237, 249, 255, 259, 265, 269, 273, 283, 287, 289, 291, 293, 297, 307 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Equally, odd numbers k such that A003415(sigma(k^2)) is odd, i.e., k^2 is in A347877. See A235991.
A square root of any hypothetical odd square x present in A005820 (triperfect numbers) would be a member of this sequence, because bigomega(x) would be even, and bigomega(3*x) would be odd. See also A347887.
LINKS
MATHEMATICA
Select[Range[1, 300, 2], OddQ[PrimeOmega[DivisorSigma[1, #^2]]] &] (* Amiram Eldar, Sep 19 2021 *)
PROG
(PARI) isA347885(n) = ((n%2)&&(bigomega(sigma(n^2))%2));
CROSSREFS
Cf. A000203, A001222, A003415, A235991, A347870, A347877, A347882, A347886 (complement among A005408), A347887 (subsequence).
Sequence in context: A020592 A295387 A263258 * A326653 A218624 A152078
KEYWORD
nonn
AUTHOR
Antti Karttunen, Sep 19 2021
STATUS
approved

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Last modified June 13 08:31 EDT 2024. Contains 373383 sequences. (Running on oeis4.)